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Common Year Calculator

Calculate if a year is a common (non-leap) year. Get day counts, weekday info, century statistics, and nearby common years instantly.

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About

A common year in the Gregorian calendar contains exactly 365 days. It occurs in every year that does not satisfy the leap year rule: divisible by 4, except centuries not divisible by 400. Approximately 303 out of every 400 years are common. Misidentifying a common year as leap (or vice versa) cascades into scheduling errors, incorrect interest accruals, and broken date arithmetic in software systems. The error is subtle because three out of four century years (1700, 1800, 1900) are common despite being divisible by 4.

This calculator applies the full Gregorian divisibility test and returns the year type, total day count, weekday alignment for January 1 and December 31, ISO week count, and the nearest neighboring common years. Results assume the proleptic Gregorian calendar for years before 1582. Note: the Julian calendar (used before the Gregorian reform) treated every fourth year as leap with no century exception, so historical dates before adoption will differ from civil records.

common year leap year calendar year calculator gregorian calendar 365 days non-leap year

Formulas

The Gregorian leap year rule determines whether a year Y is leap or common. A year is a leap year if and only if:

{
Y mod 400 = 0 → LEAPY mod 100 = 0 → COMMONY mod 4 = 0 → LEAPotherwise → COMMON

A common year is any year where the above resolves to COMMON. It has exactly 365 days. The number of days D in year Y:

D(Y) =
{
366 if leap365 if common

To find the day of the week for any date, Tomohiko Sakamoto's algorithm is used. Given a date with year y, month m, and day d:

w = (y + ⌊y4⌋ − ⌊y100⌋ + ⌊y400⌋ + t[m] + d) mod 7

Where t is the month offset table [0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4], and y is decremented by 1 if m < 3. The result w maps to 0 = Sunday through 6 = Saturday.

The number of common years in a 400-year Gregorian cycle is 400 − 97 = 303. The ratio of common to total years is 303400 = 75.75%.

Reference Data

YearTypeDaysJan 1 WeekdayDec 31 WeekdayISO WeeksNotes
1900Common365MondayMonday52Century year, not divisible by 400
2000Leap366SaturdaySunday52Century year, divisible by 400
2001Common365MondayMonday52First common year of 21st century
2019Common365TuesdayTuesday52Pre-pandemic year
2021Common365FridayFriday52 -
2022Common365SaturdaySaturday52 -
2023Common365SundaySunday52 -
2025Common365WednesdayWednesday52Current year
2026Common365ThursdayThursday53ISO long year
2027Common365FridayFriday52 -
2029Common365MondayMonday52 -
2030Common365TuesdayTuesday52 -
2031Common365WednesdayWednesday52 -
2033Common365SaturdaySaturday52 -
2034Common365SundaySunday52 -
2035Common365MondayMonday52 -
2100Common365FridayFriday52Century year, not divisible by 400
2200Common365WednesdayWednesday52Century year, not divisible by 400
2300Common365MondayMonday52Century year, not divisible by 400
2400Leap366SundayMonday52Century year, divisible by 400

Frequently Asked Questions

The Gregorian calendar adds a correction: years divisible by 100 are NOT leap unless also divisible by 400. So 1900 (divisible by 100 but not 400) is common with 365 days, while 2000 (divisible by 400) is leap with 366. This correction compensates for the fact that the tropical year is approximately 365.2422 days, not exactly 365.25.
Most common years have 52 ISO weeks. However, a common year that starts on Thursday (like 2015) has 53 ISO weeks. This is called an ISO long year. It occurs because the 365th day falls past the Thursday threshold used by ISO 8601 to assign the last week. Software that hard-codes 52 weeks per common year will miscalculate payroll periods and reporting cycles in these edge cases.
Yes. A common year has 365 days, which equals 52 weeks plus 1 day. Therefore December 31 always falls on the same weekday as January 1. A leap year (366 days = 52 weeks + 2 days) ends one weekday later than it starts.
This calculator uses the proleptic Gregorian calendar, extending Gregorian rules backward. Historical records before 1582 (or later, depending on the country - Britain adopted in 1752, Russia in 1918) used the Julian calendar where every 4th year was leap with no century exception. A year like 1500 was leap under Julian rules but would be common under Gregorian rules. Always verify which calendar system applies to your historical data.
In a typical century (like 1901-2000), there are 24 leap years and 76 common years. However, if the century year itself is common (not divisible by 400), the count shifts: centuries like 1801-1900 have only 24 leap years (1804, 1808, ..., 1896) because 1900 is common, yielding 76 common years. The 400-year cycle contains exactly 303 common years and 97 leap years.
Yes. The Julian calendar (still used by some Orthodox churches) considers every year divisible by 4 as leap, making years like 1900 and 2100 leap - not common. The Revised Julian calendar matches Gregorian rules until the year 2800. Persian (Solar Hijri) and Ethiopian calendars use entirely different intercalation rules. This calculator applies Gregorian rules exclusively.
Day-count conventions in finance (ACT/365, ACT/360, ACT/ACT) depend on the actual number of days in a year. Bond coupon accruals, loan interest, and derivatives pricing use these conventions. Using 365 days for a leap year (or 366 for a common year) produces incorrect accrued interest. The error compounds across multi-year instruments. Some conventions (like ACT/ACT ISDA) switch mid-year if a period spans both a common and leap year.